The present invention relates to a semiconductor light-emitting device which provides improved noise characteristics when operating at low output powers, and to a method for fabricating the light-emitting device, and a method for driving the same.
FIG. 14 shows a prior-art refractive index guided semiconductor laser device disclosed in Japanese Journal of Applied Physics by A. Kuramata et al., 37(1998), L1373.
For example, as shown in FIG. 14, on a substrate 101 of sapphire, formed are the following layers, each of which is formed of a III–V compound semiconductor. That is, grown by crystallization on the substrate 101 are an n-type semiconductor portion 102 containing an n-type contact layer, an active layer 103, and a p-type semiconductor portion 104 containing a p-type contact layer.
The upper portion of the p-type contact layer in the p-type semiconductor portion 104 has a ridge portion patterned in the shape of a stripe, where a p-side electrode 105 is formed on the entire surface of the ridge portion. In this structure, a region of the active layer 103 underlying the p-side electrode 105 acts as a cavity in which lasing takes place.
The n-type contact layer of the n-type semiconductor portion 102 is exposed on one side region of the p-side electrode 105, where an n-side electrode 106 is formed substantially on the entire surface of the exposed surface.
A forward drive current is applied from the p-side electrode 105 to the n-side electrode 106. When the drive current has exceeded a predetermined lasing current threshold, a laser beam is launched from one facet of the active layer 103.
Suppose that a semiconductor laser device like the one shown in FIG. 14 is used to perform a write operation on an optical disc such as a high-density digital versatile (or video) disc (HD-DVD). To use a purple laser beam in this operation, it is necessary to deliver an output of 30 mW or more. In contrast to this, it is necessary to make the output of the purple laser beam as low as 1 mW for the read operation.
However, in the read operation, there is a problem that the prior-art semiconductor laser device causes the relative intensity of noise to increase as the output decreases even when a high frequency is superimposed on the drive current. This is because a current approximately equal in magnitude to the lasing current threshold is injected to allow lasing to take place, thereby causing the relative intensity of noise to increase due to the effect of relaxation oscillation in the lasing.
In addition, lasing at approximately the same injected current as the lasing current threshold causes a characteristic of the single mode to be degraded and multi-mode components to develop, thereby increasing the relative intensity of noise.
To reduce the relative intensity of noise, it is necessary to increase the frequency of relaxation oscillation. As one of the methods that are applicable to the reduction, it is conceivable to increase the differential gain. To increase the differential gain of lasing, an optical absorption region may be formed to increase the lasing threshold.
Alternatively, the slope efficiency (differential efficiency) may be reduced to increase the current required to deliver a lasing output of approximately 1 mW, thereby setting the operating current to a value greater than the lasing threshold.
Incidentally, the facets of a cavity could be increased in reflectivity to reduce the noise of the semiconductor laser device. However, this would cause the output (optical output) of the laser beam to be reduced as well. As described above, lased light of a high output power is required for the HD-DVD device to carry out the write operation. Accordingly, this makes it impossible to employ the means for increasing the reflectivity of the facets, which leads to a reduction in optical output.
On the other hand, to allow the semiconductor laser device to provide self-pulsation, it is necessary to provide a semiconductor optical absorption layer in or near the active layer 103.
However, this raises a problem that such an optical absorption layer provided in the semiconductor laser device itself would make it difficult to provide a high output power.